The good news is that your old iPhone X case probably will fit your new iPhone XS perfectly. According to Apple’s own specs, the new XS is the exact same size as the X.
True, there are some physical differences — the antenna band gaps are in different spots, and the camera bump on back is slightly bigger. Plus, the iPhone XS has asymmetrical speaker grilles, so there could be some muffled audio or other weirdness. However, size-wise you’re most likely good.
For the iPhone XS Max? Not such good news.